Nearly all domestic and industrial wastewater effluents contain organic compounds, which can cause detrimental oxygen depletion (or demand) in waterways into which the effluents are released. This demand is due largely to the oxidative biodegradation of organic compounds by naturally occurring microorganisms, which utilize the organic material as a food source. In this process, carbon is oxidised to carbon dioxide, while oxygen is consumed and reduced to water.
Standard analytical methodologies for the determination of aggregate properties such as oxygen demand in water are biochemical oxygen demand (BOD) and chemical oxygen demand (COD). BOD involves the use of heterotrophic microorganisms to oxidise organic material and thus estimate oxygen demand. COD uses strong oxidising agents, such as dichromate or permanganate, to oxidise organic material. BOD analysis is carried out over five days and oxygen demand determined by titration or with an oxygen probe. COD measures dichromate or permanganate depletion by titration or spectrophotometry.
Despite their widespread use for estimating oxygen demand, both BOD and COD methodologies have serious technological limitations. Both methods are time consuming and very expensive, costing water industries and local authorities in excess of $1 billion annually worldwide. Other problems with the BOD assay include: limited linear working range; complicated, time consuming procedures; and questionable accuracy and reproducibility (the standard method accepts a relative standard deviation of ±15% for replicate BOD5 analyses). More importantly, interpretation of BOD results is difficult since the results tend to be specific to the body of water in question, depend on the pollutants in the sample solution and the nature of the microbial seed used. In addition, the BOD methodologies cannot be used to assess the oxygen demand for many heavily polluted water bodies because of inhibitory and toxic effects of pollutants on the heterotropic bacteria.
The COD method is more rapid and less variable than the BOD method and thus preferred for assessing the oxygen demand of organic pollutants in heavily polluted water bodies. Despite this, the method has several drawbacks in that it is time consuming, requiring 2-4 hours to reflux samples, and utilises expensive (e.g. Ag2SO4), corrosive (e.g. concentrated H2SO4) and highly toxic (Hg(II) and Cr(VI)) reagents. The use of toxic reagents being of particular environmental concern, leading to the Cr(VI) method being abandoned in Japan.
Titanium(IV) oxide (TiO2) has been extensively used in photooxidation of organic compounds. TiO2 is non-photocorrosive, non-toxic, inexpensive, relatively easily synthesised in its highly catalytic nanoparticulate form, and is highly efficient in photooxidative degradation of organic compounds.
Fox M. A. and Tien, T, Anal. Chem., (60 1988) 2278-2282 investigated the development of a photoelectrochemical detector by employing an anodically formed TiO2 electrode for use in high-pressure liquid chromatography. This photoelectrochemical detector is reported as being sensitive to oxidisable organics, such as alcohols. The electrode system developed by Fox et al had low photocatalytic efficiency of the system and is difficult to use as it cannot discriminate between the respective currents generated from the oxidation of water and organic matter.
Brown, G. N., et al., Anal. Chem., 64 (1992) 427-434 investigated the use of a photoelectrochemical detector by employing a thermally formed TiO2 electrode for use as a detector in flow injection analysis and liquid chromatography. The detector was found to be non-selective in its response to a variety of organic analytes. Brown et al found that the response of the detector varied with temperature, duration of heating, oxidative atmosphere, etching of the titanium wire electrode, amount of doping on the TiO2 detector and solvents. Similar to Fox et al this electrode system had low photocatalytic efficiency and cannot discriminate between the currents generated from the oxidation of water and organic matter.
Matthews R. W. et al., Analytica Chimica Acta 233 (1990) 171-179 (also the subject of Australian patent 597165) utilised a TiO2 photocatalytic oxidation system to determine total carbon in water samples, by placing TiO2 into a slurry or suspension, photooxidising the organic material with in the sample to evolve carbon dioxide (CO2). The evolved CO2 was measured to predict TOC value of the sample. Matthews found that the total organic carbon can be estimated from the total amount of carbon dioxide purged from photocatalytic cell.
Jiang D. et al., J. Photochem & Photobio A: Chemistry 144 (2001) 197-204 also investigated the photoelectrochemical behaviour of nanoporous TiO2 film electrodes in the photooxidation of methanol. This paper discloses a model experimental system to study the kinetics of photocatalytic water remediation on TiO2 particles. Jiang found that the photocurrent response of the electrode was greatly influenced by applied potential, light intensity, methanol concentration and pH. A linear relationship was found to exist between the photocurrent produced through the photo-oxidation of methanol and the concentration of methanol in the sample. However, as concentration of methanol increased the migration of photoelectrons across the TiO2 film and therefore photogenerated charge separation becomes a rate-limiting step, thus limiting the working range in which the linear relationship between photocurrent and concentration occurs.
Lee Kyong-Hoon et al., Electroanalysis 12, No 16 (2000) 1334-1338, investigated the determination of COD using a microfabricated Clark-type oxygen electrode and TiO2 fine particles suspended in a sample solution under photocatalytic oxidative degradation conditions. The current generated from the oxygen electrode under −800 mV applied potential was used to indicate the oxygen concentration change before and after the photooxidation. The change in oxygen concentration was then correlated to COD value of the sample.
Kim, Yoon-Chang, et al., Anal. Chem., 72 (2000) 3379-3382; Analytica Chimica Acta 432 (2001) 59-66 and Anal. Chem., 74 (2002) 3858-3864 all relate to the determination of COD using a photocatalytic oxidative degradation of organic compounds at a titanium dioxide particles. In Anal. Chem., 2000, Kim et al investigated the use of translucent poly(tetrafluoroethylene) (PTFE) membrane having fine particles of TiO2 absorbed or entrapped onto the surface of the membrane in combining with a oxygen electrode as a possible COD sensor. The immobilised TiO2 particles serve as an oxidation reagent and the analytical signal was based on the oxygen concentration measurements between the working and reference oxygen electrodes.
Calibration curves where established using sodium sulfite (Na2SO3), prior to determining COD of analytes. In this study Kim et al reports that the membrane sensor did not show good reproducibility.
In Analytica Chimica Acta 432 (2001) 59-66, Kim et al investigated the use of titanium dioxide (TiO2) beads in a photochemical column and an oxygen electrode as the sensor in determining dissolved oxygen from the photocatalytic oxidation of organic compound and thus the COD value of the analyte.
In Anal. Chem., 74 (2002) 3858-3864 Kim et al investigated the use of 0.6 mm TiO2 beads in a quartz tube in the determination of oxygen consumption from photochemical oxidation of organic compounds and subsequent calculation of COD values from the difference in the currents recorded at the reference and working oxygen electrodes.
The methods described by Lee et al and Kim et al above all utilise TiO2 as an oxidative reagent to replace the traditional reagent used in COD such as chromate salts, with the analytical signal being obtained via two traditional oxygen electrodes. There are many disadvantages of their method, which makes the practical application of the method very difficult.
To date the COD assay methodologies of the prior art are indirect in their analysis methods requiring calibration and often suffer from having low sensitivity, poor accuracy, narrow linear working ranges and/or difficult to operate. More importantly, these prior art COD assay methodologies are matrix dependent due to the low oxidation efficiency. It is an object of this invention to overcome these shortcomings.